Excessive or unwanted vibrations are common occurrences. Such vibrations can occur in many places including cars, planes, elevators, and buildings. Controlling unwanted vibrations is a diverse area with many different applications and techniques. One specific area of concern to the present inventor is vibration that may occur in the floors of buildings.
Floor vibrations may be deemed as excessive if they impair the function of the building in some way. At even very small levels, vibration can interfere with the operation of sensitive equipment. When vibrations exist at levels above human perception, they can annoy or even alarm the occupants. In offices and other quiet environments, vibration force levels as low as 0.005 g, where g is the acceleration of gravity, may yield complaints that people can't concentrate. In terms of displacement, motion having peak amplitudes less than 1 mm can be a problem. Where people are sitting or standing in more active or social environments, such as ballrooms, restaurants, and shopping malls, they may object to vibration force levels above 0.02 g. At this level, people find the vibration very noticeable. The motion is not only physically perceptible, other cues that the floor is moving, such as waves in drinks and rattling of objects on tables, become apparent. Above 0.02 g, people may start to question the safety of the structure and refuse to use the space. Even though the structure is perfectly safe at this level of vibration, the perception of danger can still exist, thus impairing the function of the building.
Excessive floor vibrations are usually caused by either equipment or by normal occupant activities. Occupant activities that have a repetitive motion, such as walking, dancing, and jumping, can cause objectionable floor motion due to resonance. Resonance exists when the frequency of the floor disturbances are the same as a natural frequency of the floor system. When resonance occurs, the amplitude of the vibration is largely affected by weight and damping characteristics of the structure. Damping is an effect that causes a dissipation of energy so that resonant vibrations cannot build to large levels. At resonance, decreases in weight and damping result in higher vibration amplitudes given a certain level of disturbance. Such decreases in weight and damping are occurring more often due to recent trends in building construction and use resulting in an increase in the instances of problem floor vibrations.
Modern construction practices are producing buildings that have natural frequencies more prone to resonance, support less weight, and have less damping than buildings of the past. Additionally, many modern offices have fewer partitions, fewer paper files, etc. and therefore fewer non-structural mechanisms for providing damping and weight. In some instances, owners of office buildings in successful service for 30 years are reporting problems that seem to be the result of a change from fully enclosed offices to cubicles. Also, structural materials have become stronger and more lightweight, thus allowing longer spans with less weight. Longer spans tend to vibrate at lower frequencies, thus resulting in natural frequencies more prone to resonance. All of these trends lead to an increased likelihood of problem floor vibrations.
Mitigating problem floor vibrations depends on the source. In the case of equipment-induced floor vibration, the best solution for reducing objectionable vibrations is in isolating the source. Reciprocating or rotating equipment, such as fans and pumps, can be placed on very flexible supports to remove the possibility of resonance with the supporting structure. This type of isolation cannot be accomplished for occupant-induced vibrations caused by activities like walking, dancing and jumping so other modifications to the structure are often necessary to reduce the vibration.
Traditional methods for improving floor vibration characteristics vary widely in cost of implementation, obtrusiveness, and effectiveness. The primary methods known to the inventor for improving floor vibration characteristics are the following: (1) adding columns, (2) adding partitions that span from the floor to the ceiling, (3) adding additional thickness to the flooring, (4) increasing the structural stiffening of the framing members, and (5) adding dampers to the structure.
Adding columns under the problem area will reduce the level of vibrations in that area. While this action is typically very effective, it is not usually a good solution because the columns will disrupt the open space. Additionally, if the columns do not run successively to the ground, they may transmit the vibration to other floors.
Full height partitions (i.e. inside walls) are also an effective way of reducing vibrations. An example of adding full height partitions would be changing an office space from a “cube-farm” type of space to a space with individual fully enclosed offices. Full height partitions are very effective in reducing problem vibrations because they provide both support and damping to the floor system. They can, however, transmit annoying vibrations to other parts of the building and interfere with the building space. Additionally, they can be costly to build, disrupt occupants during installation, and may destroy the original purpose of the building design, for example if the original intent was to have a large open space for a cube-farm.
Adding thickness (e.g. concrete) to the floor slab is also effective in reducing both the amplitude of the vibration and the natural frequency, two desirable effects with respect to human perception. The drawbacks of adding concrete thickness include cost, inadequate strength to support the additional weight, disruption in an occupied building, and the need to modify interior walls, doors and plumbing. Thus, this solution is probably not desirable except in extreme cases.
A related solution is the structural stiffening of framing members. Structural stiffening of framing members is effective in reducing walking vibrations provided the alterations do in fact substantially stiffen the floor structure. Effective stiffening of framing members, however, generally requires ceiling space below the floor structure that may not be available. Accordingly, this solution has limited applicability and also suffers from the cost and disruption of building occupants.
Perhaps the least intrusive of the traditional methods for reducing problem vibrations is the use of tuned mass dampers (TMD). TMDs are made of a weight hung from a spring attached to the vibrating structure. A damping element also connects the mass to the structure thus providing a mechanism to dissipate energy. They are “tuned” to the natural frequency of the floor system so that they are most effective in damping the expected frequency of the problem vibrations. Tuned mass dampers reduce resonant vibration amplitudes and reduce the duration of transient vibrations without a considerable change in the natural frequency of the problem vibrations. The effectiveness of a tuned mass damper is limited by the amount of additional mass that can be safely supported by the structure. Additionally, the mechanisms that provide the damping are often ineffective for very small amplitudes, thus rendering the device ineffective for all but the worst problem floors.
A more recently developed solution for the problem of unwanted floor and other structural vibrations and an area that is recognized as having significant potential for improved vibration control is known as “active vibration control”. In the context of vibration control, the term active control refers to a mechanism that uses energy from an external source to reduce vibration. Active control technology has been used in many disciplines to improve the response of dynamic systems but has had limited impact in the case of floor systems. Active control has a large advantage over passive damping techniques (e.g. tuned mass damper mentioned above). The primary advantage is that active control requires much less moving mass to effect the same degree of control. The primary disadvantage is the increase in complexity and cost of implementing and maintaining the control system.
One such prior art active control system developed by the inventor hereof is described in Engineering Journal, 4th Quarter, 1998, pp 123-127 and Journal of Structural Engineering, November 1997, pp 1497-1505 (both hereby incorporated by reference). This system uses an off the shelf actuator, velocity sensor and a PC computer based digital controller. Several disadvantages of this system are that the properties of the actuator are not optimized for maximum control effectiveness and a computer-based control loop is not very practical for actual implementation.
Additionally, some US Patents have addressed the issue of problem vibrations with a variety of passive and active control solutions. For example:
U.S. Pat. No. 6,053,269 to Patten discloses a vibration mitigation assembly for mitigating vibration of a bridge as a vehicle travels across the bridge. The system contains sensors, a controller, and multiple actuators. The actuators are hydraulic actuators and do not use a proof-mass.
U.S. Pat. No. 5,065,552 to Kobori et al. teaches an active vibration control system for adding variable damping to the frame of a structure (e.g. a building). The variable damping device includes a sensing means, a feedback control means, and a variable stiffness means. The system does not use a proof-mass actuator and is generally directed toward responding to earthquakes.
U.S. Pat. No. 5,875,589 to Lai et al. describes a method for damping floor vibrations in structures. The method comprises the attachment at multiple points on the floor of a specific type of passive vibration damping device. The device comprises an inner layer of viscoelastic material sandwiched by outer rigid layers. This approach has the significant disadvantage of requiring multiple connections to the structure and the space for the device to hang.
U.S. Pat. No. 6,223,483 to Tsukagoshi discloses a vibration damping mechanism for providing a building or other structure. The mechanism is a passive system with pivoting plates connected to the structure frame and to a viscoelastic body capable of absorbing the vibration energy.
U.S. Pat. No. 5,876,012 to Haga et al. teaches a compact vibration cancellation apparatus. The device uses various sensors, a control system, and a combination of air springs and electromagnetic actuators. The apparatus is designed to cancel the vibration of a piece of equipment and prevent it from being transferred to a floor system.
U.S. Pat. No. 6,292,967 to Tabatabai et al. discloses a tuned mass damper (TMD) design for wind-rain induced vibrations of a cable-stayed bridge. The TMD is mounted on a stay cable and comprises a viscoelastic spring system inside a cylindrical housing.
U.S. Pat. No. 6,213,444 to Yeh describes a passive vibration damper that is placed between a machine and a floor. The vibration damper has an I-shaped girder structure and includes a layer of cement and a layer of vibration absorber. The device is particularly designed for damping between a semiconductor manufacturing machine and a floor.
The prior art literature and patents describe a variety of methods for remediating excessive vibrations. However, the drawbacks to the traditional structural and passive solutions and the limited commercial success of the new active control systems illustrate the need for alternatives or improvements in controlling excessive floor vibrations. These are the primary needs addressed by the present invention. Accordingly, the following are selected objects of the present invention:
It is an object of the invention to improve the effectiveness of vibration control systems used for reducing vibrations in floors and other structures.
It is also an object of the present invention utilize a simple and therefore practical feedback controller.
It is also an object of the present invention to produce an active control system that is optimized to more effectively use the force and stroke capacity of the motor than prior art solutions while maintaining a simple feedback controller.